On overhead cam engines, the rotation of the cam shafts must be coordinated with the angle of rotation of the crankshaft in order to provide proper timing. A timing belt or a timing chain system is commonly incorporated to provide the requisite timing for the opening and closing of the valves. In both systems, proper tension in the belt or chain is critical to the operation of the engine. Proper tension reduces noise generated by the chain or belt, but more importantly, prevents tooth skip. Tooth skip can cause catastrophic failure of the engine.
For engines having a timing belt, a mechanical tensioner as described in U.S. Pat. Re. 34,543 is widely utilized. The timing belt tensioner maintains proper tension on the timing belt, even during periods of vibration, such as the vibrational experience that occurs during engine start up and shut down, when the engine passes through its resonant frequency.
For engines having a timing chain, a chain tensioning guide and some activation means to bias the guide against the running chain is utilized. The guide normally has a low friction running surface formed into a shape of either a single arc or a combination of multitude of arcs. The position of the chain tensioning guide is continually changing depending on the thermal expansion and retraction of the engine, the stretch and wear of the chain and/or the wear of the sprockets and other chain guides, as well as on the actual acting force of the above-mentioned activation means. The common activation/biasing means normally consists either a blade spring or a hydraulic tensioner consisting of a piston which is being forced towards the tensioning guide by a combination of a compression spring and the pressure of engine oil. The physical nature of these biasing means results in the actual force by which the guide is being pushed towards the chain generally diminishes as the guide is pushed further towards the chain. It is considered that in some engines it would be advantageous to keep the tensioning guide activation force constant or even increase this force when the chain stretches and the tensioning guide moves further towards the chain direction.
The timing chain is normally situated on the end of the engine and covered with a plate. Examples of timing chain tensioners and cover plates include: U.S. Pat. Nos. 6,332,441; 6,382,103; 6,352,487; 6,244,981; and 6,213,073. In the case of hydraulic tensioners, the tensioner receives pressurized oil to maintain a desired pressure on a slack portion of the chain. A relief valve is normally provided to prevent the tensioner from over tensioning the chain. Between the pressure limits, hydraulic tensioners have a tendency to “pump up” without being able to release pressure. Thus, the hydraulic tension is not readily able to release in the event of reverse tension in the chain.
It would thus be desirable to merely substitute a mechanical tension in place of a hydraulic tensioner. However, a chain assembly is normally mounted on the end of the engine and covered with a plate. As a result, there is very limited amount of space available under the cover to install a mechanical tensioner.
In order to provide long enough operational stroke for the tensioner within small space available in the modern car engines, the length of the arm eccentricity must be considerable larger than the diameter of the supporting pivot shaft. This results in the necessity to increase the frictional damping by supplying an additional frictional damping unit at the secondary pivot point between the arm and the push rod. Because of the constant, two directional damping, this tensioner design has a drawback that the damping reduces the effect of the main spring force on the tensioning guide. With the limitation on space, the tensioner is limited in the area of frictional surfaces that generate frictional damping forces that are required for proper operation of the mechanical tensioner.
Thus, it is desirable to provide mechanical tensioner that is sufficiently compact to fit within the volume between the engine block and cover plate and is capable of generating sufficient frictional damping forces for proper operation.